The present invention pertains to waste treatment facilities in general, and in particular to long-term storage of slurries.
Liquid sludge storage has been used for agricultural applications. Typically, manure from livestock is stored over a period of time, until conditions are appropriate for land application or other disposition of the stored material. It has been found convenient to store the manure in a liquid form in large open top storage tanks. As those skilled in the art will readily appreciate, the manure, which is fed into the tank in the form of a liquid/solids slurry, will begin to settle and a surface crust will start to form in a relatively short time after introduction in the storage tank. After relatively long storage times, up to six months or more, the contents of the tank must be discharged for application in a field. Due to the settling, and crust formation on the top of the tank, preparations must be made several days ahead of time to prepare the tank contents for discharge using liquid handling devices.
In waste water treatment facilities, such as municipal waste water treatment plants, sludge is processed in various liquid forms and then stored in a dried condition. However, due to environmental considerations, difficulties in handling the sludge during treatment, and other factors, there is a growing interest in storing the sludge in a liquid form.
When liquid sludge storage has been practiced in the past, the contents being stored have been continuously mixed to maintain the sludge solids in suspension. This facilitates withdrawal of sludge with relatively little preparation using liquid handling systems. However, when sludge is stored for a prolonged period of time, on the order of several months or more, the costs of maintaining sludge in a slurry form can be significant. Accordingly, there has been a recent emphasis in exploring cost savings by allowing sludge slurries in long-term storage to settle, and to mix the contents of the storage tank only prior to tank unloading.
As those skilled in the art will appreciate, a crust of substantial thickness can form on the surface of the tank and settling of solid sludge components can be quite pronounced, requiring appropriately distributed mixing energy to be applied to the tank contents so as to complete re-suspension of the solid contents of the tank. It has been found that submerged mixing devices, either of the propeller or gas type, have not been able to effectively mix tanks of larger diameter size particularly when re-suspension of solids is necessary. Fixed propeller-type mixers, either those entering the side or top of the tank provide a substantial mixing energy to the tank contents, but have been found to leave dead spots in the tank which are not mixed. Also, propeller mixers have been found effective only at certain specified water levels.
Diffused aeration systems have been used successfully on some types of mixtures, but have not been capable of re-suspending solids which have settled out of a slurry mixture, and are thus unsuitable for use with long-term sludge storage. U.S. Pat. No. 3,271,304 provides an example of a diffused aeration system.
Fixed liquid jets have been installed in storage tanks, and have been found to create a velocity sufficient to maintain solids in suspension and to re-suspend solids in the flow path. However, in practical applications, portions of the tank, oftentimes the center of the tank bottom, have been found unmixed. Also, fixed liquid jets as previously employed, have not been able to break up a crust formed on the top of the storage tanks. U.S. Pat. No. 3,586,294 shows an example of fixed liquid jets. The jets are fed from a header system located at the bottom of the storage tank, and produce counter-rotating flows. U.S. Pat. No. 4,416,549 discloses an arrangement for mounting a pump at the bottom of a storage tank, and includes a mounting arrangement for attachment to the outer wall of the tank.
Pivoting propeller mixers have been installed along tank sidewalls. In general, pivoting propeller mixers have been able to generate velocities necessary to re-suspend solids along the outer portion of the tank, but contents at the center of the tank have not been re-suspended.
Certain improvements have been provided by the arrangement of U.S. Pat. No. 4,332,484 which employs a rotatable liquid jet nozzle located at the center of a storage tank. A second nozzle is located above the water level of the tank and is manually directed to break up the top crust which forms on the tank, and to clean off the tank walls after the tank has been emptied. The centrally located rotatable nozzle is positioned adjacent the tank floor and applies velocity at a point where solids are accumulated.
In order to break up the crust formed at the upper surface of the tank contents, U.S. Pat. No. 4,512,665 provides an adjustable nozzle mounted at the top of the tank for discharging a flow downwardly on top of the crust to break up the crust in preparation for homogenization of the crust pieces by other systems.